Fix mmCIF hydrogen filtering

[OOAAHH]

Motivation

• The previous mmCIF hydrogen filter used strlen(AtomName_uf[index_uf]) == 4 as an index, which evaluates to 0 or 1 and leads to incorrect character checks.
• That bug caused some 4-character hydrogen atom names to be erroneously kept instead of being dropped.
• For mmCIF input, all hydrogen atoms should be ignored to match the intended heavy-atom based algorithms.

Description

• Replace the incorrect indexing expression with an explicit const char *atom_name and a clear check for hydrogen names.
• Skip atoms whose name starts with 'H' or whose name is digit-prefixed with 'H' as the second character using isdigit((unsigned char)atom_name[0]) && atom_name[1] == 'H'.
• Change applied to src/rnaview.c in the loop that parses _atom_site entries (around the existing hydrogen/water filtering block).